On 27 July 2011 10:45, Sergiy Nazarenko <[email protected]> wrote:
> Hi everyone!
Hi,
> I have data declaration like this:
> I try to use typeclasses,
> class GetRow a where
> hasID :: Int -> IO a
> instance GetRow MyTableOne where
> hasID myid = return [(MyTableOne 1 "name")]
> instance GetRow MyTableTwo where
> hasID myid = return [(MyTableTwo 1 "name" "path")]
The issue here seems to be that you are trying to define a class
instance for /constructors/, not types.
I don't really understand what it is you are trying to do, but perhaps you
could treat MyTableOne and MyTableTwo as distinct types.
Something like:
\begin{code}
data MyTableOne = MyTableOne Int String
data MyTableTwo = MyTableTwo Int String String
instance GetRow MyTableOne where ...
instance GetRow MyTableTwo where ...
\end{code}
You might even use a class for tables, so you can define arbitrary
table structures which all support a common interface (e.g., row fetching).
Hope this helps.
Regards,
Joachim
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